fix NPE

Author: millotp

.gitignore

@@ -0,0 +1 @@
+.DS_store

gocksnap.go

@@ -136,10 +136,13 @@ func (g *Snapshot) promptCall(req *http.Request, existingCall *Call) *Call {
 			Headers:     req.Header,
 			QueryParams: queryParams,
 		},
-		ExistingResponse: &existingCall.MockedCall,
 		finalCall:        finalCall,
 	}
 
+	if existingCall != nil {
+		g.pending.ExistingResponse = &existingCall.MockedCall
+	}
+
 	// notify SSE clients
 	for ch := range g.sseConns {
 		select {